Cleaning method and cleaning apparatus

ABSTRACT

A method of cleaning an object in a processing chamber by supplying a supercritical fluid with an additive and rinsing the object with the supercritical fluid alone includes the steps of: opening a back-pressure valve of a branch pipe branched from an additive pipe on the rinse processing; and purging a residual additive from the additive pipe by circulating the supercritical fluid alone into the additive pipe and the branch pipe.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese Patent Application JP 2006-189553 filed in the Japanese Patent Office on Jul. 10, 2006, the entire contents of which being incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cleaning method and a cleaning apparatus using a supercritical fluid, applicable to cleaning of semiconductor substrates in a semiconductor manufacturing process or cleaning of substrates in a process of manufacturing a photo-mask or a liquid-crystal display substrate, for example.

2. Description of the Related Art

In most of semiconductor apparatus manufacturing processes, for example, patterning processes for forming electrodes of desired shapes of planes, wiring patterns and contact holes are carried out by etching patterns on an electrode layer, a wiring layer, an insulating layer and the like deposited on a semiconductor substrate.

In such pattern etching, a resist film is formed on a wiring layer formed on a substrate, for example. Then, a resist mask having a resist pattern the shape of which is the same as that of a desired wiring pattern is formed by patterning the resist film. Thereafter, the resist mask is used as an etching mask and the wiring layer is etched to form a wiring pattern. After etching the wiring layer, the resist mask on the wiring pattern will be removed in a mask removing process.

In the mask removing process, the resist mask is typically removed from a substrate by immersing the substrate with the resist mask formed thereon into a resist removing solution such as a sulfate solution, an amine-based resist/polymer remover, a fluorine-based resist/polymer remover and the like during a predetermined time period.

A line width of wiring in the wiring pattern has been reduced since a semiconductor apparatus has been further integrated in recent years. In LSI (large-scale integration) circuits, a line width of wiring becomes substantially not more than 100 nm and also an aspect ratio (height/width) of wiring pattern is increased.

As a result, according to a related-art method in which the resist mask is removed by immersing the substrate into the resist removing solution, large suction force is generated at a gas-liquid interface due to large surface tension of the resist removing solution. Hence, there is a risk that a pattern with a high aspect ratio will be damaged, that is, a pattern may collapse.

A damage on a fine structure, such as a pattern collapse may occur in the process of removing a resist mask when a micromachine (MEMS: Micro Electro Mechanical Systems) having a hollow structure that includes a gap between a movable portion and a substrate is cleaned in the manufacturing process thereof. In addition, such damage may occur when a porous low-dielectric constant interlayer insulator is cleaned in the process of forming a wiring structure based on a damocene method, which is employed in the process of manufacturing a semiconductor apparatus.

That is, when an object with a fine structure is cleaned, according to a related-art cleaning method using a cleaning solution that has a large surface tension, it is difficult to clean the object without damaging the fine structure.

Japanese Unexamined Patent Application Publication No. 1-200828, Japanese Unexamined Patent Application Publication No. 1-286314, Japanese Unexamined Patent Application Publication No. 9-43857, and Japanese Unexamined Patent Application Publication No. 8-181050, for example, disclose respective supercritical technologies that use a supercritical fluid which may not cause surface tension as a resist removing solution or a cleaning solution.

When a temperature and a pressure of a material increase to be a critical temperature and a critical pressure specific to the material or higher than these conditions, the material is given intermediate properties between liquid and solid and therefore becomes a supercritical fluid which may not cause surface tension. The supercritical technologies use such supercritical fluid in a cleaning processing or the like.

In particular, carbon dioxide (CO₂) becomes a supercritical fluid at a temperature of 31° C. under a pressure of 7.38 MPa, and hence it is not difficult to be used from an industrial standpoint.

However, even when an object is cleaned using the supercritical fluid CO₂ as a cleaning solution, it is difficult for the supercritical fluid CO₂ to dissociate a resist or a resist residue from the object, so that the resist or the resist residue may not be completely removed from the object.

Accordingly, dissociation chemicals (hereinafter referred to as “additives”) to remove the resist and the resist residue from the object or a dissociation solvent thereof has been added to facilitate the removal of a resist mask from the object. Here, the dissociation solvent is a dissolution-assisting agent for an additive and used when the additive may not be highly soluble in the supercritical fluid CO₂.

U.S. Patent Unexamined Patent Application Publication No. US2002/0048731 A1 discloses a method of removing a photoresist film or a photoresist residue (hereinafter both referred to as a “resist film”) from a semiconductor substrate (hereinafter referred to as a “wafer”) by applying a supercritical fluid technology. The method is specifically described below.

First, a wafer with a resist film deposited on its surface is placed within a processing chamber of a cleaning apparatus. Then, a pressure in a pressure chamber is adjusted and a supercritical fluid CO₂ and an additive are placed within the pressure chamber. Further, the pressure in the pressure chamber is adjusted and the supercritical fluid CO₂ and the additive are introduced into the pressure chamber. The wafer is maintained in the supercritical fluid CO₂ in which the additive is dissolved, such that the supercritical fluid CO₂ including the additive may contact with the resist film until the resist film is removed from the wafer. After the resist film is removed from the wafer, the pressure is released and the wafer is cleaned.

According to US2002/0048731 A1, the method can remove not only the photoresist film and the photoresist residue but also other particles and a metal contaminated material on the wafer.

Further, if the additive is difficult to be dissolved into the supercritical fluid CO₂, in addition to the additive, a dissociation solvent may be added to the supercritical fluid CO₂ as a solvent for combining the additive and the supercritical fluid CO₂. Through those processes, a wafer and others may be cleaned using a supercritical fluid.

Japanese Unexamined Patent Application Publication No. 11-216437 discloses a technology in which, in a supercritical fluid cleaning method, a pressure is rapidly reduced by discharging a supercritical fluid from a cleaning bath to a separating bath in the cleaning process so that a flow rate of the supercritical fluid can be increased to improve cleaning effects.

FIG. 1 is a schematic diagram showing a supercritical fluid cleaning apparatus according to related art. As shown in FIG. 1, a supercritical cleaning apparatus 40 includes a substrate cleaning bath (i.e., processing chamber) 41, a main pipe (supercritical fluid pipe) 42 for supplying supercritical fluid, in which a supercritical fluid CO₂ and an additive are mixed, to the substrate cleaning bath 41, a discharge pipe 43 for discharging the supercritical fluid used for cleaning from the substrate cleaning bath 41 and additive pipes 44 and 45 for supplying the additive to the supercritical CO₂ pipe (main pipe) 42. In this example, the supercritical fluid cleaning apparatus 40 includes two systems of the additive pipes 44 and 45. Also, the pipes 42, 43, 44 and 45 are respectively provided with a supercritical CO₂ supplying valve (open/close valve) 46, a back-pressure valve 47 and additive supplying valves (open/close valves) 48 and 49.

In the cleaning apparatus 40, when an object is cleaned, such object as a semiconductor wafer is placed within the substrate cleaning bath 41. Then, the supercritical CO₂ supply valve 46, the back-pressure valve 47 and the two additive supply valves 48 and 49 are opened and the supercritical fluid in which additives 52 and 53 are added to the supercritical CO₂ 51 is supplied through the main pipe 42 to the substrate cleaning bath 41 to clean the wafer. After the wafer is cleaned, the additive supply valves 48 and 49 are closed and the supercritical CO₂ 51 alone will be supplied to rinse the wafer. After the wafer is rinsed, the supercritical CO₂ supply valve 46 is closed, a pressure in the substrate cleaning bath 41 is reduced to return to the atmospheric pressure. Subsequently, the semiconductor wafer thus cleaned is taken out from the substrate cleaning bath 41.

SUMMARY OF THE INVENTION

The above-mentioned supercritical fluid cleaning is used in order to prevent a fine structure from being damaged, that is, a pattern from collapsing, which occurs in a process of removing a resist mask. However, there still remains a problem that an additive dissolved into a supercritical fluid remaining in an end pipe is liquidized in the reduced-pressure state after the wafer is cleaned, entered into the substrate cleaning bath to wet the wafer and as a result the fine structure may be damaged. Further, since the semiconductor wafer is cleaned and rinsed with the supercritical fluid under an extremely high pressure, it is difficult to purge the remaining additives from the end pipe in the rinsing process. If the pressure in the apparatus is returned to an ordinary pressure in the condition in which the additive is insufficiently purged from the end pipe, the liquidized additive enters rapidly into the substrate cleaning apparatus to wet the semiconductor wafer.

Further, the above-described problems will be described in detail with reference to FIG. 1 and FIGS. 2A to 2D. It should be noted that broken lines in FIGS. 2A to 2D represent flows of supercritical CO₂ and that solid lines represent flows of additives and residual additives. In addition, in FIGS. 2A to 2D, open valves represent valves in the open state and solid valves represent valves in the closed state.

When a substrate is cleaned (FIG. 2A), four valves 46, 47, 48 and 49 are all open under a pressure of pipes. At that time, although the additives 52 and 53 added are pressurized in the additive pipes 44 and 45, they are generally retained in a liquid phase. The additives 52 and 53 are added to the supercritical fluid CO₂ 51 to be the supercritical state. Subsequently, the supercritical fluid to which the additives are added is used to clean the semiconductor wafer in the substrate cleaning bath 41.

Next, after the semiconductor wafer is cleaned, the additive supply valves 48 and 49 provided to the additive pipes 44 and 45 for supplying the additives are closed (FIG. 2B). At that time, part of the additives 52A and 53A flowed at the instant that the additive supply valves 48 and 49 are closed remains in the portion near the additive supply valves 48 and 49. Subsequently, the semiconductor wafer is rinsed with the supercritical CO₂ 51 alone. At that time, since the pressure of the pipe to the substrate cleaning bath 41 is high, the additives 52A and 53A left at the portions near the additive supply valves 48 and 49 remain in these portions. That is, the additives remaining in the pipes 44 and 45 to the substrate cleaning bath 41 may not be purged sufficiently.

After the semiconductor wafer is rinsed using the supercritical CO₂ 51 (FIG. 2C), the supercritical CO₂ supply valve 46 is closed, the supercritical fluid discharging valve 47 is opened to the atmospheric pressure and the supercritical fluid is discharged from the pipe 42 and the substrate cleaning bath 41 (FIG. 2D).

At that time, since the pressures in the pipe 42 and the substrate cleaning bath 41 are reduced, the additives 52A and 53A left, in the portions near the additive supply valves 48 and 49 are moved with a differential pressure in the liquidized state and entered the substrate cleaning bath 41 (see a magnified view in FIG. 2D) . As a result, the semiconductor wafer becomes wet with the liquidized additives 52A and 53A and the fine structure thereof may be damaged. Therefore, the supercritical technology may not be utilized sufficiently.

Further, if the additives are not purged sufficiently as described above, additives are mixed when another additive is added using the same pipe in the next wafer processing, causing a problem that processing intended to perform may not be carried out.

As another method of purging additives, there may be a method of causing a large amount of supercritical CO₂ 51 to flow and replace the additives 52A and 53A left in the portions near the additive supply valves 48 and 49, when the semiconductor wafer is rinsed. In this case, however, there may be problems regarding costs and a period of processing time.

It is desirable to provide a cleaning method and a cleaning apparatus in which additives remaining within pipes can be purged sufficiently after a wafer is cleaned.

According to an embodiment of the present invention, there is provided a cleaning method in which on rinse processing, a back-pressure valve of a branch pipe branched from an additive pipe is opened and a supercritical fluid is circulated into the additive pipe and the branch pipe to purge a residual additive from the additive pipe.

According to an embodiment of the present invention, there is provided a cleaning method in which on rinse processing, the back-pressure valve of the branch pipe is opened to reduce and release pressures in the additive pipes, so that the supercritical fluid is discharged through the additive pipe and the branch pipe together with the residual additive to purge the residual additive.

According to an embodiment of the present invention, there is provided a cleaning method in which on completion of the rinse processing, the back-pressure valve of the branch pipe branched from the additive pipe is opened, and residual additive is purged from the additive pipe through the branch pipe using a differential pressure between a high pressure within the processing chamber and a low pressure caused by opening the back-pressure valve.

In the cleaning method according to an embodiment of the present invention, after the rinse processing, pressure in the additive pipe is reduced and released by opening the back-pressure valves of the branch pipe, so that the supercritical fluid is discharged through the additive pipe and the branch pipe together with the residual additive to purge the residual additive.

According to another embodiment of the present invention, there is provided a cleaning apparatus in which an object in a processing chamber is cleaned by supplying a supercritical fluid with an additive and in which the object is rinsed with the supercritical fluid alone. The cleaning apparatus includes a main pipe, a discharge pipe, an additive pipe, and a branch pipe. The main pipe supplies the supercritical fluid to a cleaning bath through an open/close valve. The discharge pipe discharges the supercritical fluid from the processing chamber through a first back-pressure valve. The additive pipe supplies an additive to the main pipe through a first open/close valve and a second open/close valve used for an additive. The branch pipe is branched from an intermediate portion between the first open/close valve and the second open/close valve, including a back-pressure valve.

In the cleaning apparatus according to an embodiment of the present invention, the additive pipe is provided with the first open/close valve and the second open/close valve and the branch pipe including the back-pressure valve is derived from the intermediate portion of the two open/close valves. Accordingly, if a pressure in the additive pipe is reduced by opening the back-pressure valve of the branch pipe on the rinse processing on completion of the cleaning processing, that is, immediately before the rinse processing is ended, the residual additive is discharged from the additive pipe from the branch pipe together with the supercritical fluid to purge the residual additive.

Further, after the rinse processing is ended, if a pressure in the additive pipe is reduced by opening the back-pressure valve of the branch pipe, then the residual additive is discharged from the additive pipe through the branch pipe together with the supercritical fluid to purge the residual additive.

In the cleaning method and the cleaning apparatus according to the embodiments the present invention, the residual additive can be sufficiently purged from the additive pipe. Therefore, when a pressure in the substrate cleaning bath is reduced (when the pressure in the substrate cleaning bath is returned to an atmospheric pressure) after the cleaning processing and the rinse processing, it is possible to prevent liquidized additives from flowing into the processing chamber. As a result, such problems as a fine structure of an object being damaged by the liquidized additive and the object being wet can be prevented. Further, even in the case where another additive is added to the object using the same pipe in the subsequent processing, additives can be prevented from being mixed and hence an intended processing can be carried out.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a part of a supercritical substrate cleaning apparatus near a substrate cleaning bath according to related art;

FIGS. 2A to 2D are schematic process diagrams showing a cleaning method using a supercritical substrate cleaning apparatus according to the related art;

FIG. 3 is a schematic diagram showing a part of a supercritical fluid substrate cleaning apparatus near a substrate cleaning bath according to an embodiment of the present invention;

FIGS. 4A to 4E are schematic process diagrams showing a supercritical substrate cleaning method according to a first embodiment of the present invention; and

FIGS. 5A to 5E are schematic process diagrams showing a supercritical substrate cleaning method according to a second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to embodiments of the present invention, a separate line branched from an additive pipe for supplying an additive and connected to a back-pressure valve, that is, a branch pipe is provided. In addition, an open/close valve is provided to the additive pipe at a position between a connecting main pipe for supplying a supercritical fluid and the branch pipe.

A cleaning method according to the embodiments of the present invention, that is, a method of purging a residual additive from the additive pipe is performed using a supercritical fluid with a high pressure which is a feature thereof. According to a first method, while an object is being rinsed with the supercritical fluid, the open/close valve is opened to purge the residual additive from the additive pipe with the supercritical fluid while the pressure is being controlled using the back-pressure valve of the branch pipe branched from the additive pipe. Specifically, the back-pressure valve of the discharge pipe of the processing chamber is closed and the back-pressure valve of the branch pipe is opened to cause the supercritical fluid to flow into the additive pipe and the branch pipe to purge the residual additive together with the supercritical fluid. Further, on completion of the rinse processing including the purging of the residual from the additive pipe, the above-described open/close valve provided to the additive pipe at a position between a connecting main pipe and the branch pipe is closed. Accordingly, pressure of the pipe from the open/close valve to the back-pressure valve is returned to an ordinary pressure to further facilitate discharge of the residual additive. After the rinse processing, the back-pressure valve of the discharge pipe of the processing chamber is opened to return the pressure in the processing chamber to an ordinary pressure. Through such operations, the residual additive within the additive pipe can be prevented from flowing into the processing chamber.

According to a second method, on completion of the object being rinsed with the supercritical fluid, the back-pressure valve branched from the additive pipe is opened in the supercritical state so as to retain a sufficiently high pressure. The additive is discharged from the back-pressure valve together with the supercritical fluid using a differential pressure between a high pressure in the processing chamber and a low pressure caused by opening the back-pressure valve in the branch pipes. Further, the pipe from the open/close valve to the back-pressure valve is returned to the ordinary pressure by closing the open/close valves immediately after opening the back-pressure valve to further facilitate discharge of the additive. After the open/close valve is closed, the back-pressure valve directly connected to the processing chamber is opened to return the pressure in the processing chamber to the ordinary pressure. According to the above-mentioned operations and pressure control, the additive remaining in the additive pipe can be prevented from flowing into the processing chamber.

In the embodiments of the present invention, any material can be used as long as the material is capable of phase transition to the supercritical fluid. Preferably, carbon dioxide gas which can be phase-changed to the supercritical fluid at a substantially ordinary temperature under relatively low pressure is used as a supercritical fluid generating material. Other materials capable of phase transition to supercritical fluid have their own supercritical conditions.

In addition, in the embodiments of the present invention, at least any material selected from HF, hydroxyamine-based additive, alkanolamine-based additive, alkysil-amine-based additive, ammonium fluoride, water, IPA, methanol, ethanol, isopropanol, ethylene glycol, acetone, metyl ethyl ketone, dimethyl sulfoxide and N-methyl pyrrolidine can be used as the additive. According to the embodiments of the present invention, one kind of additive, or two or more kinds of additives mixed may be used.

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 3 is a schematic diagram showing an arrangement of a cleaning apparatus using a supercritical fluid according to an embodiment of the present invention.

As shown in FIG. 3, a cleaning apparatus according to the embodiment of the present invention, that is, a supercritical cleaning apparatus 1 includes a processing chamber 2, a main pipe 3, additive pipes 4 and 5, and a discharge pipe 6. An object is placed in the processing chamber (hereinafter referred to as “substrate cleaning bath” where wafers are cleaned according to the embodiment) 2. The main pipe 3 supplies supercritical CO₂ 21 to the substrate cleaning bath 2 as a supercritical fluid. The additive pipes 4 and 5 are connected to the main pipe 3 and supply two kinds of additives 22 and 23 respectively, according to this embodiment. The discharge pipe 6 discharges a supercritical fluid used in the substrate cleaning bath 2. The main pipe 3 is provided with an open/close valve 7 used for supplying the supercritical CO₂, the discharge pipe 6 is provided with a back-pressure valve 10 and the additive pipes 4 and 5 are provided with first open/close valves 8 and 9 used for supplying additives.

In addition, according to the embodiment of the present invention, second open/close valves 15 and 16 are provided to the additive pipes 4 and 5 at a position between a connecting main pipe 3 and the first open/close valves 8 and 9. A branch pipe 11 is derived from an intermediate portion between the first open/close valve 8 and the second open/close valve 15 of one additive pipe 4. Also, a branch pipe 12 is derived from an intermediate portion between the first open/close valve 9 and the second open/close valve 16 of the other additive pipe 5. The two branch pipes 11 and 12 are provided with back-pressure valves 13 and 14, respectively.

The back-pressure valve 10 of the discharge pipe 6 and the back-pressure valves 13 and 14 of the branch pipes 11 and 12 are valves with which pressure can be controlled.

Next, a first embodiment of a cleaning method for cleaning an object, for example, a semiconductor wafer (hereinafter referred to as a “wafer”) using the supercritical cleaning apparatus 1 according to the embodiment will be described with operation processes.

FIGS. 4A to 4E are respective diagrams showing those operation processes. It should be noted that open valves represent valves in the open state and that solid valves represent valves in the closed state in FIGS. 4A to 4E. Also, thick broken lines represent flows of supercritical CO₂ and thick solid lines represent flows of additives and residuals thereof.

First, a wafer is accommodated and placed within the substrate cleaning bath 2. Next, as shown in FIG. 4A, cleaning processing is carried out. Specifically, the open/close valve 7 and the back-pressure valve 10 are opened so as to retain a supercritical state and a supercritical CO₂ 21 heated and pressurized is supplied to the substrate cleaning bath 2 through the main pipe 3. Further, in the state in which the back-pressure valves 13 and 14 of the branch pipes 11 and 12 are closed, the first open/close valves 8 and 9 and the second open/close valves 15 and 16 of the additive pipes 4 and 5 are opened. Accordingly, first and second additives 22 and 23 are supplied through the additive pipes 4 and 5 to be added to the supercritical CO₂ 21 in the main pipe 3. A supercritical fluid containing the first and second additives 22 and 23 dissolved into the supercritical CO₂ 21 is supplied to the substrate cleaning bath 2 to clean the wafer.

Next, on completion of the cleaning process using the supercritical fluid containing the dissolved additives, as shown in FIG. 4B, rinse processing is carried out while the supercritical fluid containing the dissolved additives is being substituted with a pure supercritical fluid. Specifically, the closed states of the back-pressure valves 13 and 14 of the branch pipes 11 and 12 are maintained and the first open/close valves 8 and 9 of the additive pipes 4 and 5 are closed to stop the supply of the first and second additives 22 and 23. In that state, the supercritical CO₂ 21 alone is supplied and the wafer placed in the substrate cleaning bath 2 is rinsed with the supercritical CO₂ 21. During the rinse processing, the wafer is rinsed while the supercritical fluid is discharged with the pressure thereof adjusted using the back-pressure valve 10. For example, since CO₂ becomes supercritical at a temperature of 31° C. under a pressure of 7.38 MPa, the back-pressure valve 10 is opened in conditions higher than the above-described temperature and pressure. In the rinse processing, while the additive pipes 4 and 5 at the downstream side of the first open/close valves 8 and 9 are also rinsed, additives 22A and 23A are partly left at portions near the first open/close valves 8 and 9.

Next, during the rinse processing, as shown in FIG. 4C, residual additives 22A and 23A are purged from the additive pipes 4 and 5 immediately before the end of the rinse processing according to this embodiment. Specifically, the back-pressure valves 13 and 14 of the branch pipes 4 and 5 are opened during the rinse processing. When the back-pressure valves 13 and 14 are opened, the back-pressure valve 10 of the discharge pipe 6 is closed simultaneously. At that time, the back-pressure valves 13 and 14 are opened, while maintaining the rinsing supercritical fluid within the substrate cleaning bath 2 at a temperature and a pressure to retain the supercritical state. As a result, the supercritical CO₂ 21 is discharged through the additive pipes 4 and 5 and the branch pipes 11 and 12. At the same time, the residual additives 22A and 23A are also discharged with the supercritical CO₂ 21 and the inside of the additive pipes 4 and 5 are purged.

Next, as shown in FIG. 4D, after the inside of the additive pipes 4 and 5 are sufficiently substituted with the supercritical CO₂ 21, the second open/close valves 15 and 16 of the additive pipes 4 and 5 are closed. As a result, the rinse processing is ended. At that time, it is preferable that the back-pressure valves 13 and 14 of the branch pipes 11 and 12 should be in the open state. When the back-pressure valves 13 and 14 of the branch pipes 11 and 12 are opened, the pipes from the second open/close valves 15 and 16 to the back-pressure valves 13 and 14 are returned to an ordinary pressure. Therefore, it is possible to facilitate the discharge of a small amount of the additives remaining in the additive pipes 4 and 5.

Next, as shown in FIG. 4E, the inside of the substrate cleaning bath 2 is returned to the ordinary pressure (atmospheric pressure). Specifically, the open/close valve 7 of the main pipe 3, the first open/close valves 8 and 9 and the second open/close valves 15 and 16 of the additive pipes 4 and 5 are closed, and then the back-pressure valve 10 of the discharge pipe 6 is opened. At that time, since the second open/close valves 15 and 16 are closed, even when a small amount of additives is left in the additive pipes 4 and 5, the remaining additives can be prevented from being liquidized and entering the substrate cleaning bath 2. Subsequently, the supply of the supercritical CO₂ 21 is stopped, the pressure in the substrate cleaning bath 2 is returned to be the ordinary pressure, and the substrate cleaning processing is ended.

According to the first embodiment of the cleaning method, the residual additives can be purged from the additive pipes 4 and 5 during the rinse processing. Therefore, when the pressure in the substrate cleaning bath 2 is returned to be an ordinary pressure after the rinse processing is ended, the residual additives can be prevented from flowing into the substrate cleaning bath 2.

Next, a second embodiment of a cleaning method for cleaning an object using the supercritical cleaning apparatus 1 shown in FIG. 3 will be described with the operation processes.

FIGS. 5A to 5E are diagrams showing those operation processes. It should be noted that open valves represent valves in the open state and that solid valves represent valves in the closed state in FIGS. 5A to 5E. Also, thick broken lines represent flows of supercritical CO₂ and thick solid lines represent flows of additives and residual additives.

First, a wafer is accommodated and placed within the substrate cleaning bath 2. Next, as shown in FIG. 5A, cleaning processing is carried out. Specifically, the open/close valve 7 and the back-pressure valve 10 are opened so as to retain a supercritical state and a supercritical CO₂ 21 heated and pressurized is supplied to the substrate cleaning bath 2 through the main pipe 3. Further, in the state in which the back-pressure valves 13 and 14 of the branch pipes 11 and 12 are closed, the first open/close valves 8 and 9 and the second open/close valves 15 and 16 of the additive pipes 4 and 5 are opened. Accordingly, first and second additives 22 and 23 are supplied through the additive pipes 4 and 5 to be added to the supercritical CO₂ 21 in the main pipe 3. A supercritical fluid containing the first and second additives 22 and 23 dissolved into the supercritical CO₂ 21 is supplied to the substrate cleaning bath 2 to clean the wafer.

Next, on completion of the cleaning process using the supercritical fluid containing the dissolved additives, as shown in FIG. 5B, rinse processing is carried out while the supercritical fluid containing the dissolved additives is being substituted with a pure supercritical fluid. Specifically, the closed states of the back-pressure valves 13 and 14 of the branch pipes 11 and 12 are maintained and the first open/close valves 8 and 9 of the additive pipes 4 and 5 are closed to stop the supply of the first and second additives 22 and 23. In that state, the supercritical CO₂ 21 alone is supplied and the wafer placed in the substrate cleaning bath 2 is rinsed with the supercritical CO₂ 21. During the rinse processing, the wafer is rinsed while the supercritical fluid is discharged with the pressure thereof adjusted using the back-pressure valve 10. For example, since CO₂ becomes supercritical at a temperature of 31° C. under a pressure of 7.38 MPa, the back-pressure valve 10 is opened in conditions higher than the above-described temperature and pressure. In the rinse processing, while the additive pipes 4 and 5 at the downstream side of the first open/close valves 8 and 9 are also rinsed, additives 22A and 23A are partly left at portions near the first open/close valves 8 and 9.

Next, as shown in FIG. 5C, the open/close valve 7 of the main pipe 3 is closed. Simultaneously, the back-pressure valve 10 of the discharge pipe 6 is closed and the rinse processing is ended.

Next, as shown in FIG. 5D, after the rinse processing is ended, the back-pressure valves 13 and 14 of the branch pipes 11 and 12 are opened to purge the residual additives 22A and 23A from the additive pipes 4 and 5. Specifically, on completion of the rinse processing using the supercritical CO₂ 21, the back-pressure valves 13 and 14 of the branch pipes 4 and 5 are opened simultaneously, while the substrate cleaning bath 2 is retained at sufficiently high pressure with the supercritical CO₂. Since CO₂ becomes the supercritical state at a temperature of 31° C. under a pressure of 7.38 MPa, the back-pressure valves 13 and 14 are opened in conditions higher than the above-described temperature and pressure. As a result, using a differential pressure between the high pressure within the substrate cleaning bath 2 and the atmospheric pressure caused by opening the back-pressure valves 13 and 14, the residual additives 22A and 23A within the additive pipes 4 and 5 and the supercritical CO₂ 21 are discharged from the back-pressure valves 13 and 14, and the pipes are purged.

Next, as shown in FIG. 5E, the second open/close valves 15 and 16 of the additive pipes 4 and 5 are closed immediately after the back-pressure valves 13 and 14 are opened. Accordingly, even if a small amount of additives is left in the additive pipes 4 and 5, the remaining additives can be prevented from being liquidized and entering the substrate cleaning bath 2. Subsequently, pressures in the pipes from the first open/close valves 8 and 9 to the back-pressure valves 13 and 14 are returned to be an ordinary pressure and discharge of the additives is further facilitated.

After the second open/close valves 15 and 16 are closed, the back-pressure valve 10 of the discharge pipe 6 for the substrate cleaning bath 2 is opened and the pressure in the substrate cleaning bath 2 is returned to be an ordinary pressure to end the cleaning processing.

According to the second embodiment of the cleaning method, the residual additives 22A and 23A can be purged using the differential pressure between the high pressure maintained within the substrate cleaning bath 2 and the atmospheric pressure caused by opening the back-pressure valves 13 and 14 after the rinse processing is ended. Accordingly, when the pressure in the substrate cleaning bath 2 is returned to an ordinary pressure on completion of the rinse processing, the residual additives can be prevented from entering the substrate cleaning bath 2.

According to the first embodiment, the number of the cleaning processes is reduced because the additive pipes 4 and 5 can be purged during rinse processing. According to the second embodiment, the cost is reduced because the additive pipes 4 and 5 can be purged after the supply of the supercritical fluid CO₂ 21 is stopped, which prevents a large amount of supercritical fluid CO₂ from being wasted. Further, the pressure in the substrate cleaning bath 2 is retained at a high pressure in order to maintain the supercritical state and the second embodiment uses the fact that the inside of the substrate cleaning bath 2 is retained at the high pressure. Hence, characteristics of the supercritical cleaning can sufficiently be used in the second embodiment.

As described above, according to the supercritical cleaning method and the supercritical cleaning apparatus of the embodiments, the additives in the supercritical fluid remaining in the end pipes are prevented from being liquidized at the reduced pressure stage after the wafer is cleaned and rushing into the substrate cleaning bath to wet the wafer and damage the fine structure. Accordingly, the supercritical fluid substrate cleaning can be performed sufficiently, in which the supercritical fluid is used in order to prevent the fine structure from being damaged by the contact of liquid such as pattern collapse occurred in the resist mask removal process.

Further, according to the supercritical substrate cleaning apparatus of the embodiment, even if another additive is added from the same pipe in the next wafer processing, the additives can be prevented from being mixed and the processing can be carried out in accordance with the purposes.

According to the above-mentioned embodiments, back-pressure valves capable of controlling a pressure are used to adjust the pressure, however, the additives can be purged while the pressure in the apparatus can be controlled using the open/close valves.

In addition, according to the above-mentioned embodiments, two additive pipe systems are provided, however, it is possible to increase the number of systems in accordance with the number of additives used by similarly setting up the back-pressure valves and the open/close valves.

It should be noted that alternatively in the above-mentioned embodiments a wafer may be cleaned using a supercritical fluid including an additive (chemical) dissolved into an organic solvent (for example, methanol). Subsequently, rinse processing using the organic solvent (for example, methanol) and rinse processing using the supercritical fluid alone may be performed at the subsequent rinse processing.

The cleaning method according to the embodiments of the present invention is applied to cleaning of a semiconductor substrate. It should be appreciated that the present invention is not limited to those embodiments and that the cleaning method according to an embodiment of the present invention can be applied to cleaning of other objects, such as cleaning of a photo-mask and a liquid-crystal display substrate.

It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof. 

1. A method of cleaning an object in a processing chamber by supplying a supercritical fluid with an additive and rinsing the object with the supercritical fluid alone, comprising the steps of: opening a back-pressure valve of a branch pipe branched from an additive pipe on the rinse processing; and purging a residual additive from the additive pipe by circulating the supercritical fluid alone into the additive pipe and the branch pipe.
 2. A method of cleaning an object according to claim 1, further comprising the steps of: providing the additive pipe joined to a main pipe for supplying the supercritical fluid with a first open/close valve at a side of the main pipe and a second open/close valve for supplying the additive, deriving the branch pipe including the back-pressure valve from an intermediate portion between the first open/close valve and the second open/close valve, and opening the first open/close valve and the back-pressure valve on the rinse processing and closing the second open/close valve to purge the residual additive.
 3. A method of cleaning an object in a processing chamber by supplying a supercritical fluid with an additive and rinsing the object with the supercritical fluid alone, comprising the steps of: opening a back-pressure valve of a branch pipe branched from an additive pipe on the rinse processing; and purging a residual additive from the additive pipe through the branch pipe using a differential pressure between a high pressure within the processing chamber and a low pressure caused by opening the back-pressure valve.
 4. A method of cleaning an object according to claim 3, further comprising the steps of: providing the additive pipe joined to a main pipe for supplying the supercritical fluid with a first open/close valve at a side of the main pipe and a second open/close valve used for supplying the additive, deriving the branch pipe including the back-pressure valve from an intermediate portion between the first open/close valve and the second open/close valve, and opening the first open/close valve and the back-pressure valve on completion of the rinse processing and closing a back-pressure valve of a discharge pipe of the processing chamber and the second open/close valve to purge the residual additive.
 5. An apparatus for cleaning an object in a processing chamber by supplying a supercritical fluid with an additive and rinsing the object with the supercritical fluid alone, comprising: a main pipe for supplying the supercritical fluid to the processing chamber through an open/close valve; a discharge pipe for discharging the supercritical fluid from the processing chamber through a first back-pressure valve; an additive pipe for supplying an additive to the main pipe through a first open/close valve and a second open/close valve used for supplying the additive; and a branch pipe derived from an intermediate portion between the first open/close valve and the second open/close valve, the branch pipe including a second back-pressure valve.
 6. A cleaning apparatus according to claim 5, wherein the second back-pressure valve is opened on the rinse processing, and the supercritical fluid alone is circulated into the additive pipe and the branch pipe to purge a residual additive from the additive pipe.
 7. A cleaning apparatus according to claim 5, wherein the second back-pressure valve is opened on completion of the rinse processing, and a residual additive is purged from the additive pipe through the branch pipe using a differential pressure between a high pressure within the processing chamber and a low pressure caused by opening the second back-pressure valve. 